Cleanup method for optics in immersion lithography using sonic device

ABSTRACT

A megasonic immersion lithography exposure apparatus includes an optical transfer chamber for containing an exposure liquid, at least one megasonic plate operably engaging said optical transfer chamber for propagating sonic waves through the exposure liquid, and an optical system provided adjacent to said optical transfer chamber for projecting light through a mask and said exposure liquid and onto a wafer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Division of U.S. patent application Ser. No. 11/237,651 filedSep. 29, 2005, which is a Continuation of International Application No.PCT/US2004/010309 filed Apr. 2, 2004, which claims the benefit of U.S.Provisional Patent Application No. 60/462,556 filed Apr. 11, 2003 andU.S. Provisional Patent Application No. 60/482,913 filed Jun. 27, 2003.The disclosures of these applications are hereby incorporated byreference herein in their entireties.

BACKGROUND

This invention relates to an immersion lithography system and moreparticularly to methods, as well as systems, for cleaning up the opticalelement that contacts and absorbs water in the process of immersionlithography.

Immersion lithography systems, such as disclosed in W099/49504, which isherein incorporated by reference for describing the general backgroundof the technology as well as some general considerations relatedthereto, are adapted to supply a liquid into the space between aworkpiece such as a wafer and the last-stage optical element of anoptical system for projecting the image of a reticle onto the workpiece.The liquid thus supplied improves the performance of the optical systemand the quality of the exposure.

The liquid to be supplied may be water for light with wavelength of 193nm although different liquids may be necessary for light with otherwavelengths. Because the last-stage optical element of the opticalsystem is exposed to the liquid, there is a possibility that some of theliquid may be absorbed. This possibility is particularly high if thelast-stage optical element of the optical system is a lens becausecalcium fluoride is a common lens material for lithography systems whileit is a hygroscopic material that is capable of absorbing water from thesurrounding environment.

The absorbed water may cause several problems. First, it may degrade theimage projected by the lens by changing the refractive properties of thelens or by causing the lens to swell to thereby change the geometry ofthe lens. Second, it may cause long-term degradation of the lens due tochemical effects.

Conventional air-immersion exposure lithography systems require theoptical elements to be made detachable for maintenance work such as whenthey are cleaned. It is a cumbersome and time-consuming operation,however, to remove an optical element and to reset it after it iscleaned or to exchange an optical element for a new one.

It is therefore an object of this invention to provide systems andmethods for periodically removing the water from the lens such that theamount of absorbed water will not reach a critical level and thedegradation of the image and the long-term damage to the lens can beprevented.

It is another object of the invention to provide systems and methods formaking the maintenance of the optical element of an immersionlithography apparatus easier and thereby improve the useful lifetime ofthe optical element.

SUMMARY

Immersion lithography apparatus of this invention may include a reticlestage arranged to retain a reticle, a working stage arranged to retain aworkpiece, an optical system including an illumination source and anoptical element opposite the workpiece for projecting an image patternof the reticle onto the workpiece by radiation from the illuminationsource while defining a gap between the optical element and theworkpiece, and a fluid-supplying device for providing an immersionliquid between and contacting both the optical element and the workpieceduring an immersion lithography process. The apparatus also includes acleaning device to clean the optical element. The term “cleaning” willbe used throughout this disclosure to mean both removing immersionliquid that has been absorbed into the optical element and removingdirt, debris, salts and the like from the optical element.

Many different kinds of cleaning devices may be used within the scope ofthis invention. For example, the cleaning device may use a cleaningliquid having affinity to the immersion liquid to be contacted with theoptical element. If the immersion liquid is water, ethanol may serve asthe cleaning liquid. As another example, the cleaning device may includea heat-generating device for heating the optical element and/or a vacuumdevice for generating a vacuum condition on the optical element.

Ultrasonic vibrations may be used for removing the absorbed liquid. Anultrasonic vibrator such as a piezoelectric transducer may be attachedto the housing for the optical element or placed opposite the opticalelement such that the vibrations may be transmitted to the opticalelement through a liquid maintained in the gap.

Alternatively, cavitating bubbles may be used for the removal of theabsorbed liquid. A pad with fins may be used to generate cavitatingbubbles in a liquid maintained in the gap between the pad and theoptical element.

According to another embodiment of the invention, the nozzles throughwhich the immersion liquid is supplied into the gap between theworkpiece and the optical element may be used to alternately supply acleaning liquid by providing a flow route-switching device such as aswitch valve.

With a system and method of this invention, the cleaning procedurebecomes significantly easier and faster because there is no need todetach the optical element to be cleaned and the cleaning processimproves the useful lifetime of the optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in conjunction with the followingdrawings of exemplary embodiments in which like reference numeralsdesignate like elements, and in which:

FIG. 1 is a schematic cross-sectional view of an immersion lithographyapparatus to which methods and systems of this invention may be applied;

FIG. 2 is a process flow diagram illustrating an exemplary process bywhich semiconductor devices are fabricated using the apparatus shown inFIG. 1 according to the invention;

FIG. 3 is a flowchart of the wafer processing step shown in FIG. 2 inthe case of fabricating semiconductor devices according to theinvention;

FIG. 4 is a schematic drawing showing a side view of a portion of theimmersion lithography apparatus of FIG. 1;

FIG. 5 is a schematic side view of a portion of another immersionlithography apparatus having an ultrasonic transducer attached so as toserve as its cleaning device;

FIG. 6 is a schematic side view of a portion of another immersionlithography apparatus having a piezoelectric cleaning device below itsoptical system;

FIG. 7 is a schematic diagonal view of an example of a piezoelectricdevice;

FIG. 8 is a schematic side view of a portion of another immersionlithography apparatus having two mutually attached piezoelectric planarmembers as the cleaning device;

FIG. 9 is a schematic side view of a portion of another immersionlithography apparatus having a bubble-generating pad as the cleaningdevice; and

FIG. 10 is a schematic side view of a portion of another immersionlithography apparatus having a switching device incorporated in thefluid-supplying device.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an immersion lithography apparatus 100 to which cleaningmethods and systems of this invention may be applied.

As shown in FIG. 1, the immersion lithography apparatus 100 comprises anilluminator optical unit 1 including a light source such as an excimerlaser unit, an optical integrator (or homogenizer) and a lens andserving to emit pulsed ultraviolet light IL with wavelength 248 nm to bemade incident to a pattern on a reticle R. The pattern on the reticle Ris projected onto a wafer W coated with a photoresist at a specifiedmagnification (such as ¼ or ⅕) through a telecentric light projectionunit PL. The pulsed light IL may alternatively be ArF excimer laserlight with wavelength 193 nm, F₂ laser light with wavelength 157 nm orthe i-line of a mercury lamp with wavelength 365 nm. In what follows,the coordinate system with X-, Y- and Z-axes as shown in FIG. 1 isreferenced to explain the directions in describing the structure andfunctions of the lithography apparatus 100. For the convenience ofdisclosure and description, the light projection unit PL is illustratedin FIG. 1 only by way of its last-stage optical element (such as a lens)4 disposed opposite to the wafer W and a cylindrical housing 3containing the rest of its components.

The reticle R is supported on a reticle stage RST incorporating amechanism for moving the reticle R in the X-direction, the Y-directionand the rotary direction around the Z-axis. The two-dimensional positionand orientation of the reticle R on the reticle stage RST are detectedby a laser interferometer (not shown) in real time and the positioningof the reticle R is affected by a main control unit 14 on the basis ofthe detection thus made.

The wafer W is held by a wafer holder (not shown) on a Z-stage 9 forcontrolling the focusing position (along the Z-axis) and the tiltingangle of the wafer W. The Z-stage 9 is affixed to an XY-stage 10 adaptedto move in the XY-plane substantially parallel to the image-formingsurface of the light projection unit PL. The XY-stage 10 is set on abase 11. Thus, the Z-stage 9 serves to match the wafer surface with theimage surface of the light projection unit PL by adjusting the focusingposition (along the Z-axis) and the tilting angle of the wafer W by theauto-focusing and auto-leveling method, and the XY-stage 10 serves toadjust the position of the wafer W in the X-direction and theY-direction.

The two-dimensional position and orientation of the Z-stage 9 (and hencealso of the wafer W) are monitored in real time by another laserinterferometer 13 with reference to a mobile mirror 12 affixed to theZ-stage 9. Control data based on the results of this monitoring aretransmitted from the main control unit 14 to a stage-driving unit 15adapted to control the motions of the Z-stage 9 and the XY-stage 10according to the received control data. At the time of an exposure, theprojection light is made to sequentially move from one to another ofdifferent exposure positions on the wafer W according to the pattern onthe reticle R in a step-and-repeat routine or in a step-and-scanroutine.

The lithography apparatus 100 described with reference to FIG. 1 is animmersion lithography apparatus and is hence adapted to have a liquid(or the “immersion liquid”) 7 of a specified kind such as water fillingthe space (the “gap”) between the surface of the wafer W and the lowersurface of the last-stage optical element 4 of the light projection unitPL at least while the pattern image of the reticle R is being projectedonto the wafer W.

The last-stage optical element 4 of the light projection unit PL may bedetachably affixed to the cylindrical housing 3 and is designed suchthat the liquid 7 will contact only the last-stage optical element 4 andnot the cylindrical housing 3 because the housing 3 typically comprisesa metallic material and is likely to become corroded.

The liquid 7 is supplied from a liquid supply unit 5 that may comprise atank, a pressure pump and a temperature regulator (not individuallyshown) to the space above the wafer W under a temperature-regulatedcondition and is collected by a liquid recovery unit 6. The temperatureof the liquid 7 is regulated to be approximately the same as thetemperature inside the chamber in which the lithography apparatus 100itself is disposed. Numeral 21 indicates supply nozzles through whichthe liquid 7 is supplied from the supply unit 5. Numeral 23 indicatesrecovery nozzles through which the liquid 7 is collected into therecovery unit 6. The structure described above with reference to FIG. 1is not intended to limit the scope of the immersion lithographyapparatus to which the cleaning methods and devices of the invention areapplicable. In other words, the cleaning methods and devices of theinvention are applicable to immersion lithography apparatus of manydifferent kinds. In particular, the numbers and arrangements of thesupply and recovery nozzles 21 and 23 around the light projection unitPL may be designed in a variety of ways for establishing a smooth flowand quick recovery of the immersion liquid 7.

A method embodying this invention of removing the portion of the liquid7 such as water absorbed by the last-stage optical element 4 made of ahygroscopic material, as well as dirt, debris, etc., is explained nextwith reference to FIGS. 1 and 4. After the wafer W is exposed with lightfrom the illuminator optical unit 1 through the light projection unit PLin the presence of the liquid 7 as shown in FIG. 1, the liquid 7 isremoved from underneath the light projection unit PL and a cleaningdevice 30 is brought into contact with the last-stage optical element 4as shown in FIG. 4. In the case of a portable kind, as shown in FIG. 4,the cleaning device 30 may be placed on the Z-stage 9 or theaforementioned wafer holder thereon, as shown in FIG. 4, in place of thewafer W.

Different types and kinds of cleaning devices 30 can be used for thepurpose of this invention. As a first example, the cleaning device 30may be a container containing a liquid (“cleaning liquid”) having astrong affinity to the immersion liquid 7 that is absorbed by theoptical element 4. If the immersion liquid 7 is water, the cleaningdevice 30 may contain ethanol because ethanol has a strong affinity towater. Any cleaning liquid may be used provided it has a sufficientlystrong affinity to the liquid to be removed and does not damage theoptical element 4 or its coating. The bottom surface of the opticalelement 4 is soaked in the cleaning liquid for a period of timesufficiently long to reduce the level of the absorbed immersion liquid.The cleaning device 30 is removed thereafter and the optical element 4is ready to be exposed to the liquid 7 again.

As another example, the cleaning device 30 may contain a heat-generatingdevice and/or a vacuum device (not separately shown). The combination ofheat and vacuum on the surface of the optical element 4 causes theabsorbed liquid to undergo a phase change into vapor, or to evaporatefrom the surface. The reduction in liquid density on the surface of theoptical element 4 draws the liquid 7 that is absorbed more deeply in theelement 4 to the surface of the optical element 4.

FIG. 5 shows a third example in which use is made of an ultrasonictransducer (or ultrasonic vibrator) 32 attached to the housing 3 of thelight projection unit PL. As the ultrasonic transducer 32 (such as apiezoelectric transducer) is activated, pressure waves are generated andpropagated, serving to clean the surface of the optical element 4.

During the cleaning operation in FIG. 5, the gap adjacent to the opticalelement 4 is filled with the immersion liquid 7. In this case, thesupply and recovery nozzles can continue to supply and collect theimmersion liquid 7, or the supply and recovery nozzles can stopsupplying and collecting the immersion liquid 7. Also during thecleaning operation, the optical element 4 can face a surface of wafer W,a surface of the Z-stage 9, or a surface of another assembly.

FIG. 6 is a fourth example using a vibratory tool 34 placed below theoptical element 4 to be cleaned. The tool 34 may be shaped like thewafer W with thickness more or less equal to that of the wafer W, orabout 0.5-1 mm, and may be made entirely of a piezoelectric materialsuch that its thickness will fluctuate when activated. As the tool 34 isplaced below the optical element 4, like the wafer W as shown in FIG. 1,and the gap between the optical element 4 and the tool 34 is filled withthe liquid 7, pressure waves are generated in the immersion liquid 7 toclean the optical element.

During the cleaning operation of FIG. 6, the gap adjacent to the opticalelement 4 is filled with the immersion liquid 7. In this case, thesupply and recovery nozzles can continue to supply and collect theimmersion liquid, or the supply and recovery nozzles can stop supplyingand collecting the immersion liquid 7. In another example, the vibratortool 34 may be a ultrasonic transducer attached to the wafer holder on aZ-stage 9, or another assembly.

FIG. 7 shows another tool 36, structured alternatively, having aplurality of piezoelectric transducers 38 supported by a planarsupporting member 39.

FIG. 8 shows still another example of a cleaning device having twoplanar members 40 of a piezoelectric material attached in a face-to-facerelationship and adapted to oscillate parallel to each other and out ofphase by 180° with respect to each other. As a result, these members 40,attached to each other, will vibrate in the transverse directions, asshown in FIG. 8 in a very exaggerated manner. The vibration has nodepoints at constant intervals where the members 40 are not displaced. Themembers 40 are supported at these node points on a supporting member 41.As voltages are applied to these members 40 so as to cause thevibrations in the mode described above, ultrasonic pressure waves arethereby generated and propagated through the liquid 7, and the opticalelement 4 is cleaned, as desired.

FIG. 9 shows still another example of a cleaning device that cleans theoptical element 4 by creating cavitating bubbles. Cavitating bubblestrapped and energized by ultrasound are high-temperature, high-pressuremicroreactors and intense energy released by the implosive compressionof the bubbles is believed to rip molecules apart. The example shown inFIG. 9 is characterized as comprising a pad 43 with fins protrudingupward and rapidly moved horizontally as shown by an arrow below theoptical element 4 with a bubble-generating liquid 17 filling the gap inbetween (structure for moving the pad 43 not being shown). As the pad 43is thus moved, the fins serve to stir the liquid 17 and to generatecavitating bubbles that in turn serve to clean the optical element.

FIG. 10 shows a different approach to the problem of cleaning thelast-stage optical element 4 by applying a cleaning liquid on its bottomsurface by using the same source nozzles 21 used for supplying theimmersion liquid 7. For this purpose, a switch valve 25 is insertedbetween the supply nozzle 21 and the liquid unit 5 such that theimmersion liquid 7 and the cleaning liquid can be supplied selectivelythrough the supply nozzle 21.

It is again noted that the cleaning methods and systems according tothis invention are applicable to immersion lithography apparatus ofdifferent kinds and types, for example, having different numbers ofsource nozzles. A switch valve as described above need not necessarilybe provided to each of the source nozzles but may be provided to a groupof the source nozzles.

The wafer W itself or a pad 18 of a suitable kind may be placed belowthe optical element 4 to provide a suitable gap in between when thecleaning liquid is thus supplied through the supply nozzles 21. Thisembodiment of the invention is advantageous because the same nozzlesalready present for supplying the immersion liquid can be utilized forthe cleaning process.

Although various methods have been separately described above, they maybe used in combinations, although that is not separately illustrated inthe drawings. For example, the pad 43 with fins shown in FIG. 9 may beused instead of the pad 18 of FIG. 10. In other words, the examplesdescribed above are not intended to limit the scope of the invention,and many modifications and variations are possible within the scope ofthis invention. For example, a polishing pad similar to one used inchemical mechanical polishing may be used for this purpose. The cleanupprocedure shown in FIGS. 4-10 may be carried out with ultraviolet light.The light may irradiate the optical element 4. The light may be normalexposure light from the illuminator optical unit 1 or some other lightof an appropriate wavelength for the purpose of the cleanup. In anotherexample, the ultraviolet light for the purpose of the cleanup may beused without the cleanup procedure shown in FIGS. 4-10, and may be usedunder a condition in which the gap adjacent to the optical element 4 isfilled with the immersion liquid 7 from the liquid supply unit 5. Allsuch modifications and variations that may be apparent to a personskilled in the art are intended to be within the scope of thisinvention.

Any of the above described cleaning methods for removing immersion fluidabsorbed by the last-stage optical element also may be used to removesalts, deposits, dirt and debris that may have accumulated. The termcleaning therefore refers to both of these phenomena.

FIG. 2 is referenced next to describe a process for fabricating asemiconductor device by using an immersion lithography apparatusincorporating a cleaning device embodying this invention. In step 301the device's function and performance characteristics are designed.Next, in step 302, a mask (reticle) having a pattern is designedaccording to the previous designing step, and in a parallel step 303, awafer is made from a silicon material. The mask pattern designed in step302 is exposed onto the wafer from step 303 in step 304 by aphotolithography system such as the systems described above. In step 305the semiconductor device is assembled (including the dicing process,bonding process and packaging process), then finally the device isinspected in step 306.

FIG. 3 illustrates a detailed flowchart example of the above-mentionedstep 304 in the case of fabricating semiconductor devices. In step 311(oxidation step), the wafer surface is oxidized. In step 312 (CVD step),an insulation film is formed on the wafer surface. In step 313(electrode formation step), electrodes are formed on the wafer by vapordeposition. In step 314 (ion implantation step), ions are implanted inthe wafer. The aforementioned steps 311-314 form the preprocessing stepsfor wafers during wafer processing, and selection is made at each stepaccording to processing requirements.

At each stage of wafer processing, when the above-mentionedpreprocessing steps have been completed, the following post-processingsteps are implemented. During post-processing, initially, in step 315(photoresist formation step), photoresist is applied to a wafer. Next,in step 316 (exposure step), the above-mentioned exposure device is usedto transfer the circuit pattern of a mask (reticle) onto a wafer. Then,in step 317 (developing step), the exposed wafer is developed, and instep 318 (etching step), parts other than residual photoresist (exposedmaterial surface) are removed by etching. In step 319 (photoresistremoval step), unnecessary photoresist remaining after etching isremoved. Multiple circuit patterns are formed by repetition of thesepreprocessing and post-processing steps.

While a lithography system of this invention has been described in termsof several preferred embodiments, there are alterations, permutations,and various substitute equivalents which fall within the scope of thisinvention. There are many alternative ways of implementing the methodsand apparatus of the invention.

1. A method used in a liquid immersion lithography apparatus, the methodcomprising: exposing a substrate by projecting an image onto thesubstrate through an exposure liquid in an exposure operation; andgenerating pressure waves in a cleaning liquid between a member of thelithography apparatus and a movable component of the lithographyapparatus which is movable below the member, by using a transducerdisposed above the movable component, to remove a contaminant from themember in a cleaning operation in which the substrate is not in contactwith the cleaning liquid, wherein the transducer is mounted apart fromthe movable component and does not move with the movable component, andthe substrate is moved below the member in the exposure operation andthe member does not face the substrate in the cleaning operation.
 2. Themethod according to claim 1, wherein the cleaning liquid includes theexposure liquid.
 3. The method according to claim 1, wherein the memberincludes an optical element of an optical system by which the image isprojected onto the substrate in the exposure operation.
 4. The methodaccording to claim 1, wherein the substrate is held on a substrateholding portion of the movable component in the exposure operation. 5.The method according to claim 4, wherein the movable component is forcontrolling a focus position and a tilting angle of the substrate in theexposure operation.
 6. The method according to claim 4, wherein themovable component includes a stage.
 7. The method according to claim 1,wherein the contaminant includes debris.
 8. The method according toclaim 1, wherein the contaminant includes dirt.
 9. The method accordingto claim 1, further comprising irradiating the member with ultravioletlight in the cleaning operation.
 10. The method according to claim 1,wherein the transducer includes a piezoelectric transducer.
 11. Themethod according to claim 1, wherein the member is in contact with theexposure liquid in the exposure operation.
 12. The method according toclaim 11, wherein the exposure liquid covers only a portion of a surfaceof the substrate in the exposure operation.
 13. The method according toclaim 12, wherein the exposure liquid is retained between the member andthe substrate in the exposure operation.
 14. The method according toclaim 13, wherein the member includes an optical element of an opticalsystem by which the image is projected onto the substrate in theexposure operation.
 15. The method according to claim 14, wherein thetransducer is attached to a housing of the optical system.
 16. Themethod according to claim 1, wherein the exposure liquid is suppliedfrom an inlet onto the substrate in the exposure operation in which thesubstrate is moved below the inlet and the movable component is movablebelow the inlet.
 17. The method according to claim 1, wherein thecleaning liquid is supplied from an inlet disposed above the movablecomponent.